In most land-based applications, navigation is often aided by in-place infrastructure such as GPS, radio beacons or a priori maps. Navigation and mapping underwater is difficult because among other things, wide-coverage underwater GPS-equivalents do not exist and large portions of the sea bed are still unexplored.
Current techniques for underwater navigation use publicly available bathymetry maps. However, these maps are relatively coarse and unsuitable for precision navigation. Other sonar-based navigation systems rely on positioning schemes that use the sonar data itself. For example, on-the-fly acoustic feature-based systems attempt to use sonar to detect naturally occurring landmarks. Other solutions to the navigation problem include deploying low-cost transponders in unknown locations thereby enabling range-based measurements between the vehicle and transponder beacon. However, these transponders are often deployed at locations that are at great distances from each other, and often only partially observable because of the range-only information. Thus, these technologies are unsuitable for navigation across small vehicle paths.
It is often desirable to be able to navigate terrain (whether on land or underwater) in a vehicle equipped appropriately with sensors that allow the vehicle to navigate the terrain relative to a prior map of the terrain. Holographic navigation is a technique that allows for precise navigation of a vehicle equipped with sonar sensor array(s) relative to a prior map. Quite often, these vehicles are autonomous underwater vehicles (AUVs) or unmanned aerial vehicles (UAVs).
A synthetic aperture sonar (or radar) array on such a vehicle generates at least one image that can be compared to an image associated with the map of the terrain and may be processed by a computer system on the vehicle to navigate the vehicle in relation to the prior map of the terrain. For instance, SAS arrays enable coherent correlation between sonar signals, whether generated by the synthetic aperture, or not.
Existing holographic navigation systems suffer from a plurality of deficiencies, including the amount of power consumed by, the size, and the shape of these systems. In some existing systems, for example, the performance of a holographic navigation algorithm degrades substantially as transmitter frequencies increase and wavelengths decrease.
Furthermore, accurately localizing a transmitter based on received emissions is a known problem. Direct path observations can be used with an array or antenna to get a bearing to an object. Multiple direct path observations can be used for triangulation while waveguide models can be used for ranging from a single location. However, these techniques are all limited in precision. Extremely long arrays (such as SOSUS) can focus on the location of an emitter object with fairly high precision provided that it is in the near field, but are extremely expensive. Hence, there is not an existing method of passively localizing an emitter with extremely high precision using a low cost receiver.
Accordingly, there is a need for high precision and low cost navigation systems, particularly for underwater applications.